The present invention relates to novel 6-substituted naphthalene-2-carboxylic acids represented by the formula (I): ##STR2## wherein X represents a hydroperoxy group or a hydroxy group, and a process for producing the same.
The 6-substituted naphthalene-2-carboxylic acids are useful compounds as intermediates for producing a polymer of 6-hydroxynaphthalene-2-carboxylic acid, which has attracted attention in recent years as a high polymeric material for producing molded articles such as fibers, etc., because the polymer of 6-hydroxynaphthalene-2-carboxylic acid shows excellent high elasticity, high tensile strength and high heat resistance. Namely, 6-hydroxynaphthalene-2-carboxylic acid, which is a monomer for the above polymer, is easily obtainable by subjecting the 6-substituted naphthalene-2-carboxylic acid to acid decomposition.
As the method for producing 6-hydroxynaphthalene-2-carboxylic acid, the following methods have been hitherto known.
(i) A method comprises reacting potassium salt of .beta.-naphthol with gaseous carbon dioxide at a high temperature under a high pressure (refer to U.S. Pat. Nos. 1,593,816; 4,287,357; 4,345,095; 4,329,494 and 4,345,094),
(ii) A method comprises reacting potassium salt of .beta.-naphthol with gaseous carbon dioxide in a medium of high boiling point at a high temperature and under a high pressure (refer to Japanese Patent Applications Laying-Open (KOKAI) No. 57-95939 (1982) and No. 58-99436 (1983)), and
(iii) A method comprises reacting 6-bromo-2-naphthol with carbon monoxide in methanol (refer to Japanese Patent Application Laying-Open (KOKAI) No. 57-91955 (1982)).
However, the above-mentioned methods have the respective defects as follows.
(a) The production cost in the methods (i), (ii) and (iii) are high due to the expensive production apparatus, because it is necessary to carry out the reaction of the methods (i) and (ii) at a high temperature range of 260.degree. to 280.degree. C. and also the reaction of the method (iii) under a high pressure of up to 70 kg/cm.sup.2.
(b) Large amount of by-product such as .beta.-naphthol in the methods of (i) and (ii) complicates the processes because of the necessity of removal of .beta.-naphthol in an after-treatment.
(c) Since the production of an unnecessary isomer of 6-hydroxynaphthalene-2-carboxylic acid such as 3-hydroxynaphthalene-2-carboxylic acid is inevitable, there are problems of the difficulty in separating the isomers and of the reduction in the yield of the objective compound.
In addition to these defects, the objective compound is produced in a low yield by the above-mentioned known methods, for instance, around 26.5% in the method (i), around 45% in the method (ii) and around 37% in the method (iii) and accordingly, none of the three methods are suitable for manufacturing the objective product in a large scale.
In an attempt to pursue industrially advantageous methods for producing 6-hydroxynaphthalene-2-carboxylic acid, the inventors have succeeded in the synthesis of methyl 6-(2-hydroperoxy-2-propyl)naphthalene-2-carboxylate as an intermediate for producing 6-hydroxynaphthalene-2-carboxylic acid. Namely, methyl 6-(2-hydroperoxy-2-propyl)naphthalene-2-carboxylate can be obtained in a high yield by oxidizing methyl 6-isopropylnaphthalene-2-carboxylate in the presence of a cobalt or manganese salt of an organic acid as a catalyst (refer to Japanese Patent Application Laying-Open (KOKAI) No. 60-243063 (1985)).
However, in order to oxidize the isopropyl group of 6-isopropylnaphthalene-2-carboxylic acid, it is necessary to use the compound in which the carboxylic group at 2-position has been esterified, as the starting material.
Accordingly, as a result of further study of the oxidation of isopropyl group of 6-isopropylnaphthalene-2-carboxylic acid, the present inventors have found novel compounds, 6-(2-hydroperoxy-2-propyl)naphthalene-2-carboxylic acid and 6-(2-hydroxy-2-propyl)naphthalene-2-carboxylic acid obtained by directly oxidizing the isopropyl group of 6-isopropylnaphthalene-2-carboxylic acid without preliminarily esterifying the carboxylic group at 2-position of 6-isopropylnaphthalene-2-carboxylic acid. Furthermore, the present inventors have found that 6-hydroxynaphthalene-2-carboxylic acid can be obtained profitably at a high purity by subjecting each or both of the above-mentioned two novel compounds to acid decomposition, and based on these findings, the present invention has been attained.